1. Field of Invention
The invention relates to an exhaust gas recirculation device that partially recirculates exhaust gas discharged from an internal combustion engine into an intake pipe thereof, and particularly, to a failure diagnosis system for diagnosing a failure in the exhaust gas recirculation device.
2. Description of Related Art
An exhaust gas recirculation (hereinafter referred to “EGR”) device for partially recirculating the exhaust gas discharged from the internal combustion engine into an intake pipe thereof has been widely employed in the internal combustion engine for a vehicle so as to reduce nitrogen oxides (NOx) contained in the exhaust gas and to improve fuel efficiency. When failure in the operation of the EGR device is caused by abnormality in an operation of an EGR valve or an EGR pipe, the amount of generated NOx is likely to be increased. The failure in the EGR device, however, hardly influences the performance of the EGR device itself. This may keep the vehicle operator from becoming aware of the abnormality in the EGR device. Accordingly a large amount of NOx may be discharged to atmosphere, leading to air pollution.
The technology for detecting the failure in the EGR device as aforementioned has been proposed in publications, for example, JP-A-5-1624 or JP-A-7-4320 so as to solve the aforementioned problem.
JP-A-5-1624 discloses a failure diagnosis system for an exhaust gas recirculation control device in which the accuracy for detecting abnormality in the EGR device is improved upon its ON/OFF operations. The failure diagnosis system for the exhaust gas recirculation control device is provided with a recirculation pipe that recirculates the exhaust gas discharged from the internal combustion engine into an intake pipe, a recirculation valve that controls a flow rate of the exhaust gas flowing through the recirculation pipe, a recirculation valve passage area control unit that controls a passage area of the recirculation valve, an operation condition detection unit that detects an operation condition of the internal combustion engine, a memory unit that stores a first detection value detected by the operation condition detection unit when the passage area of the recirculation valve which is brought into a first state where the passage area is relatively large by the recirculation valve passage area control unit, a memory unit that stores a second detection value detected by the operation condition detection unit when the passage area of the recirculation valve which is brought into a second state where the passage area is relatively narrow or zero by the recirculation valve passage area control unit, and a determination unit for diagnosing the failure based on at least the first and the second detection values. The determination unit serves to diagnose the failure in a predetermined area except a high load area where the detection value detected by the operation condition detection unit indicates larger loss in the recirculation pipe, and a low load area where the flow rate of the exhaust gas flowing through the recirculation pipe is small.
In the failure diagnosis system for the exhaust gas recirculation control device, upon detection of the abnormality in the EGR device by tuning ON/OFF thereof, the failure determination area is limited depending on the load. This makes it possible to detect the failure in ON state of the EGR device and in an area where the detection value such as the difference in the intake manifold pressures between ON state and OFF state of the EGR device is large. The failure is detected only when the flow rate of the exhaust gas recirculated by the EGR device is equal to or lower than a predetermined value. Accordingly, fluctuation in the torque upon ON/OFF operations of the EGR device may be reduced.
JP-A-7-4320 discloses an exhaust gas recirculation control device capable of accurately diagnosing abnormality in an exhaust gas recirculation device. The exhaust gas recirculation control device is provided with a recirculation pipe that recirculates the exhaust gas discharged from the internal combustion engine into an intake pipe, a recirculation pipe operation device that opens or closes the recirculation pipe, an intake air condition unit that detects a condition of air flowing through the intake pipe, a first control unit that controls an opening degree of the recirculation pipe operation device between a first value and a second value that is smaller than the first value, a first change amount detection unit that detects a change amount of the condition of air flowing through the intake pipe when the recirculation pipe operation device is operated by the first control unit, a first determination unit that determines whether a detection result of the first change amount detection unit is smaller than a predetermined amount, a second control unit that changes the opening degree of the recirculation pipe operation device by the difference in the opening degree larger than the one that has been changed by the first control unit when it is determined that the change amount detected by the first change amount detection unit is smaller than the predetermined amount, a second change amount detection unit that detects a change amount of the condition of air flowing through the intake pipe when the operation of the recirculation pipe operation device is changed by the second control unit, and a second determination unit that determines the exhaust gas recirculation device as being abnormality when a detection result of the second change amount detection unit is smaller than the predetermined value.
In the exhaust gas recirculation control device, when the EGR device is in a normal operation state, the operation state of the EGR device is generally detected as being normal by the first determination unit. In the case where the EGR rate obtained during detection of abnormality in the EGR device is small, the operation state may be detected as being abnormal by the first determination unit contrary to the fact that the actual operation state is normal. In the aforementioned case, the EGR rate is increased, and then the determination is made with respect to abnormality in the operation state by the second determination unit. The operation state at this stage may at least be determined as being normal. This makes it possible to prevent the false determination with respect to the operation state from being made as abnormal contrary to the actual normal operation state. When the operation state of the EGR device is abnormal, it may be determined as being abnormal with certainty by the second determination unit. In the aforementioned exhaust gas recirculation control device, the determination with respect to the abnormality in the EGR device will be made again at the increased EGR rate only when the first determination unit does not determine the EGR device as being in the normal operation state. Accordingly this makes it possible to prevent increase in the exhaust gas caused by unnecessarily increasing the EGR rate.
In the failure diagnosis system for an EGR control device disclosed in JP-A-5-1624 or the EGR control device disclosed in JP-A-7-4320, when the EGR device is normally operated, the exhaust gas is recirculated into the intake manifold. Accordingly the pressure change or change in the flow rate at the intake manifold is detected upon ON/OFF operations of the EGR device, based on which the abnormality diagnosis with respect to the abnormality in the EGR device may be performed.
When the intake air flow rate in the intake manifold changes in the normal operation state of the EGR device, it is difficult to determine whether the change in the pressure or the flow rate detected at the intake manifold has been caused by the failure in the EGR device or by the change in the intake air flow rate. Accordingly the failure diagnosis with respect to the EGR device cannot be performed accurately. For example, in the case where the operation timing of the intake valve is controlled by the variable valve timing mechanism that controls the intake/exhaust valve variably, the pressure or the flow rate at the intake manifold may be changed.